It is known that a client such as a mobile phone synchronizes data with a server. The client regularly makes inquiries about whether there is any data that has been changed and, if there is updated data, downloads the data. In the background art, the server keeps a change log to manage whether there is any data that has been changed. For example, according to PTL 1, in order to avoid sending and receiving a large quantity of data in single data synchronization, an update log of a second database is stored and synchronization is initiated when a number of records in the update log is greater than a threshold.